Test-Driven Development

Inputs

Preferred inputs — provide as many as exist:

• docs/user-stories/0001-*.md — defines the actor, behaviour, and BDD scenarios to implement
• docs/user-stories/0001-*-TRI.md — architecture, data model, API design, NFRs, and testing strategy for this feature
• LANGUAGE.md — canonical domain terms; use for test naming and interface vocabulary
• MAP.md — system map; use to understand component boundaries and avoid duplication

Philosophy

Core principle: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.

Good tests are integration-style: they exercise real code paths through public APIs. They describe what the system does, not how it does it. A good test reads like a specification - "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.

Bad tests are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior.